1. Field of the Invention
The present invention relates to analog integrated circuits. In particular, the present invention relates to voltage booster circuits such as are used in data converters.
2. Description of Related Art
Integrated switched-capacitor filter circuits, such as those used in the analog modulator of Delta-Sigma analog-to-digital converters (ADCs), use metal-oxide-semiconductor (MOS) transistors as switches to perform sampling tasks and to achieve the transfer of charge between the various capacitors within the filter. The switches profoundly affect circuit performance.
The switch's; on-resistance is a function of the MOS gate to source potential difference. When used at low power supply voltages, it becomes difficult to turn the switch on--since the limited supply voltage makes it difficult to increase the gate-to-source potential above the threshold voltage of the MOS device. Small switches exhibit a high on resistance. This results in reduced circuit bandwidth and limits the maximum usable sample rate. This is particularly undesirable in Delta-Sigma ADCs, where over-sampling at a high ratio (100 or more) is common. However, small switches do give best DC performance--a result of low charge injection errors from the small channel and stray capacitances. Larger switches can improve the circuit bandwidth and allow faster sampling rates. However, the accompanying increase in charge injection from using larger switches results in higher offsets and higher offset drift errors for the ADC, which are critical issues in high resolution converters.
A common technique to improve the performance of switched capacitor circuits under low-voltage operation is to use a voltage boosting circuit on the sampling clock signals to increase the available gate drive voltage to the NMOS switch transistors. This can dramatically reduce the switch on resistance, increasing the maximum sample rate without a corresponding increase in the switch size and a reduction of the DC performance. However, the boosted circuit may no longer be able to operate at high supply voltages for the following reason. If used at a high supply voltage, the boosted gate voltage can be so high that it stresses the switch and driver devices and results in reduced reliability--even catastrophic failure. As a result, existing designs are either targeted at high or at low supply voltage operation and optimized accordingly. This limits the applicability of a particular data converter design to a narrow range of possible power supply voltages.
Therefore, it is desirable to have a method and apparatus to provide a voltage boosting circuit which can be automatically reconfigured according to the power supply voltage.